1. Field of the Invention
This invention relates to dye transfer thermal printer receiver sheets, and more particularly to selectively energizing heating elements for improving image quality.
2. Description of the Prior Art
In a thermal printer shown in FIG. 1, a picture signal corresponding to an object viewed by a television camera 1 is fed to an analog/digital converting circuit 2. This analog/digital converting circuit 2. This analog/digital circuit 2 digitizes the picture signal in synchronization with the scanning on the camera picture plane and feeds a digital signal having 8 bits corresponding to the dye density tone quality to a control circuit 3 being subject to program control. The control circuit 3 generates a matrix-like picture pattern having pixels (256.times.256, for example) in response to the digital signal input from the analog/digital circuit 2.
A key board 5 for generating character patterns and a character pattern generating circuit 6 (a character generating ROM) are also connected to the control circuit 3. When character information is input from the key board 5, the control circuit 3 generates a character code corresponding to the input character information. This character code is fed to the circuit 6 and causes the corresponding character pattern to be read from the character pattern generating circuit 6. Thus, the control circuit 3 serves to make a printing pattern or matrix of pixels which is a combination of characters and pictures and causes it (as shown in FIG. 2) to be stored in a memory circuit 7. The memory circuit 7 comprises a memory having a capacity of 512.times.512.times.8 bits, with the density or tone level of each pixel of the printing pattern corresponding to the 8 bits mentioned above. In other words, the density level can be selected from 256 different levels. The present invention makes use of the fact that a number of the lower levels (say the first 40) will not be sufficient to cause dye to transfer. This is described in detail in the Description of the Preferred Embodiments. The character pattern contained in the memory circuit 7 can be monitored through a monitor 8.
The control circuit 3 controls an address control circuit 10 so that a printing pattern contained in the memory circuit 7 is sequentially fed to a thermal print head (TPH) driving circuit 11 line by line. The driving circuit 11 converts the 8-bit dye density data for each pixel of the printing pattern to a pulse length or pulse number and selectively energized heating elements in a thermal print head 12 (TPH). The amount of heat generated causes the transfer of dye from a dye donor or film in direct correspondence to the amount of heat generated by each heating element. Each heating element corresponds to an image pixel in the receiver. For a more complete description of a thermal printer and circuitry for energizing heating elements, see U.S. Pat. No. 4,621,271 to Brownstein.
In a case where a printing pattern to be printed by the thermal printer contains narrow lines surrounding character or ruled lines for a table, it may happen that only one image pixel in a printed line correponds to the narrow line has a high density level (e.g. 255) as shown in FIG. 3(a) while adjacent image pixels are not printed. In such a case, a single heating element of the TPH 12 is energized and some of the heat produced will be absorbed by the adjacent heating elements as shown by the temperature (TEM) profile in FIG. 3(b). The temperature of the energized heating element corresponding to the single pixel to be printed is lowered due to heat transferred to adjacent heating elements as compared with the case of printing a plurality of contiguous pixels in a picture pattern. Consequently, in a case of printing narrow lines, the printing dye intensity of some pixels may be lower than desired.